1. Field of the Invention
The present invention relates to a method and apparatus for measuring beam end displacements by determining the position of the beam cross section with time utilizing embedded optical fibers and a photodetector x-y grid sensor.
2. Description of the Prior Art
Damping loss factor determination for composite materials is currently conducted using a variety of sensors. The sensors are used to determine the displacement of the composite specimen during vibration as a function of time. Two of the more commonly used sensors for determining the displacement versus time information are the noncontact eddy current probe and the attached accelerometer. Both of these devices produce an electrical signal which can be related to a displacement.
There are two problems that are often overlooked when using these sensors. First, the analysis, using the equations of motion of a beam in bending, assumes certain conditions at the ends of the beam are known. Experimentally, it is assumed that the displacements are measured at the tip of the specimen. The sensors, however, cannot be positioned at the exact end of the test piece, but are instead located close to the end and do not therefore measure actual tip deflection. Secondly, these sensors detect displacements, or an acceleration, at a particular cross section of the specimen. The assumption made in the data reduction is that the cross section moves uniformly with one dimensional vibratory motion, i.e. that plane sections remain plane. This assumption is valid for certain materials, such as metals, which may be both homogenous and isotropic. However, for other materials, such as composites which can be fabricated with fibers oriented in many directions, this assumption will no longer suffice since bending-twisting couplings can arise which will result in two dimensional vibratory motion.
The recent advances in laser technology and optical fibers have been combined to offer a highly accurate method for measurement and testing. Optical fibers, which may be encased in various specialized jacket materials, have increasingly demonstrated their use as highly sensitive sensors. A large variety of fiber optic sensors are currently in use and include acoustic, magnetic, rate of rotation, acceleration, electric, electric current, trace vapor, pressure, and temperature sensors. They are being applied to hydrophones, magnetometers, gyroscopes, accelerometers, spectrophones, communication cables, and many other devices. These devices exhibit numerous advantages, the most important of which are geometric flexibility, immunity to electromagnetic pulses, large bandwidth, and great sensitivity, i.e. ability to detect extremely low signal levels and small signal level changes. Although many fiber optic systems are currently under investigation, the two types of fiber optic sensors of relevance, are either phase modulated or intensity modulated sensor devices.
The phase modulated applications depend primarily upon force field induced length changes and strain induced refractive index changes which causes transduction, or phase shifting, as the lightwave travels throughout the sensing length of the optical fiber and which can be detected using an interferometer apparatus. A spiral array of optic fiber wound around a mandrel and interferometry has been used to sense stress waves (acoustic emission and ultrasound) within transparent materials. Others have used similar lengths of single mode fiber in both the signal and reference arms of a Mach-Zehnder interferometer for the detection of ultrasonic waves in solids. A rectangular grid array of 50 individual fibers, arranged to give 10 mm. squares, has been described to determine the two dimensional stress distribution produced by the point loading of a simply supported square plate. Each of the fibers in this array yielded a measurement of stress integrated along the fiber length which when plotted on an x-y normalized stress graph indicated the position of any stress concentration. A monomode optical fiber interferometer which detects interference between light reflected from the end face of the fiber and light reflected back to the surface from a vibrating surface has also been described. An evanescent wave coupler was used to balance the light levels between fibers and to divert the return beam to a photodetector. Surface displacements of as small as 5 nm. were measured in this manner. Still others have demonstrated a fiber optic strain gauge using change in the optical path length due to the deformation of a fringe which results when the reference and signal beams are combined to produce interference patterns. When this strain gauge was applied to the surface of 300 mm. long by 5 mm. thick cantilever beam, a displacement of 10.28 m. was indicated by one fringe.
All of the above described configurations are applications of the phase modulated type sensor and represent primary measurement devices. The electric current and trace vapor sensors are also of this same type but are secondary devices in that they rely on the measurement of the magnetic field or the temperature in the former case and the sound associated with the absorption of light in the latter case. Phase modulated fiber optic sensors may thus be characterized by their required use of coherent light sources, single mode fibers, and relatively complex optical and electronic circuitry.
The intensity modulated type fiber optic sensors, on the other hand, depend primarily on an optical source of constant intensity which is ordinarily acted upon by an external force field so as to modulate the output light intensity incident on the photodetector, thereby modulating the output voltage of the photodetector. A conceptually simple use of an intensity type sensor is an application wherein the optical fiber is adhered directly to the surface of the specimen. This configuration may be used to detect fracture by being broken when the adjacent crack develops. The break can be detected either at the point of the light emission from the open fractured optical fiber end, or may be detected at a location remote from the break by monitoring the attenuation of the light level. Others have used the transit time of a circulating light pulse in long multimode optical fiber to measure the absolute length of the fiber and its length variations. This system operates as a transit-time oscillator and was proposed for use in a digital read out fiber optic strain gauge. An interferometric optical fiber strain measurement technique has also been described wherein the emphasis on the work is on remote measurement of slowly varying microstrains. This device is read by a frequency modulated laser source and is compatible with passive sensor multiplexing. The reference against which the mechanical displacement is measured is the modulation frequency rather than the optical frequency. The potential resolution of mechanical displacement using this device is 10 nm.
All of the above described applications of the intensity modulated type sensor may be characterized by their employment of relatively simple optics and circuitry. An incoherent optical source, such as a light emitting diode (LED) or a high intensity incandescent source may be used, together with one or two multimode fibers as links between the sensor, the modulating element and the detector. While all of these devices have the advantage of being much simpler in design and operation, they are however, less sensitive than the phase modulated fiber optic sensors. Achievable sensitivities, expressed in terms of minimum detectable displacements for intensity type mechanical motion sensors, lie in the order of 10.sup.-10 to 10.sup.-7 m., as compared to a lower limit of 10.sup.-14 m. achievable with the phase modulated type fiber optic sensors.